1. Field of the Invention
This invention relates to molded case circuit breakers and, more particularly, to a power drive mechanism, mechanically interlocked with a circuit breaker operating handle, which allows the circuit breaker handle to be controlled from a remote location. The power drive mechanism includes means for damping mechanical oscillations of the mechanism.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well-known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are generally used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and a relatively high level short circuit condition. An overload condition is normally 200 to 300 percent of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000 percent or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers generally include at least one pair of separable main contacts which may be operated manually by way of an operating handle extending outwardly from the circuit breaker case or automatically in response to an overcurrent or high level short circuit condition. In the manual mode and one automatic mode of operation, the separable main contacts are opened by an operating mechanism which, in turn, is actuated by either an electronic trip unit in the automatic mode or the operating handle in the manual mode. In another automatic mode of operation, magnetic repulsion forces, generated between the stationary and movable contacts during relatively high level overcurrent conditions, can also cause the main contacts to be separated, independently of the operating mechanism.
The operating mechanism generally includes an overcenter toggle mechanism having relatively strong operating springs. In the manual mode of operation, sufficient force must be applied to the operating handle to overcome the opposing force of the operating springs from either an on or off position to the overcenter position. Once past the overcenter position, the force of the operating springs assist in driving the operating handle to the on or off position.
In some applications, a circuit breaker may be used primarily as a switch. In such applications, the circuit breakers are provided with either a solenoid operator or a motor operator, mechanically interlocked to the operating handle for alternatively driving the operating handle to the on position or off position. Examples of circuit breakers utilizing solenoid operators are disclosed in U.S. Pat. Nos. 4,553,115 and 4,642,726, assigned to the same assignee as the present invention. In such circuit breakers, a single solenoid operator is provided to drive the operating handle. However, such solenoid operators are relatively slow acting and generally not used when relatively fast switching is required as in the case of, for example, synchronizing a motor generator set with a live electrical bus. In such an application, the circuit breaker is disposed between the generator and the live electrical bus. When the generator is synchronized with the electrical bus, the circuit breaker must be closed relatively quickly. In such applications, a motor operator is generally used for switching the circuit breaker. Such motor operators generally include a high speed, high torque electric motor, mechanically coupled to the circuit breaker operating handle. In such applications, the operating springs in the circuit breaker operating mechanism work against the motor operator until the overcenter toggle mechanism in the circuit breaker passes the overcenter position. Once past the overcenter position, the operating springs assist the electric motor to cause the separable main contacts to either open or close. The motor operator also assists the operating springs in driving the operating handle to either the on or off position. However, once the operating handle reaches either the on or off position, it is necessary to turn the motor off and reset (e.g., reverse the direction of rotation) the motor for the next operation. Generally, limit switches are disposed at the on position and the off position for this purpose. These limit switches are actuated by a carriage assembly, mechanically coupled to the shaft of the motor. Due to the high speed and the large force at which the carriage assembly operates near the end of travel, a substantial amount of force is applied to these limit switches. Such force can necessitate the use of relatively expensive, heavy duty limit switches.
Furthermore, the carriage assembly oscillates after it engages a limit switch due to the force and speed of the carriage assembly causing the limit switch contacts to bounce. Since the limit switch controls the operation and the direction of rotation of the motor, this can cause erratic operation of the motor operator, thus preventing the circuit breaker from being accurately controlled.